None.
Not Applicable.
This invention relates to an improved support structure for a bearing installed in a low pressure (LP) section of a steam turbine.
Referring to the drawings, a section of a turbine T is shown in FIG. 1. The section shown in FIG. 1 is a portion of a low pressure section of the turbine and includes an axial flow LP bearing support or housing B of a conventional design. A bearing (not shown) installed in the housing supports the hub of a rotor (also not shown) to which are mounted turbine blades or buckets (also not shown). A metal shell or casing H comprises a hollow cylinder. Axially mounted within the shell is a cylindrically shaped housing B closed at one end and to which pipes P are attached for venting and sealing the bearing assembly, all as is well known in the art. The housing is supported within the shell by struts S.
Four struts S1-S4 are horizontal plates two of which extend inwardly from opposite sides of inside wall W1 of the shell. Another strut S5 extends vertically from the underside of the housing to the bottom of the inside wall surface. Finally, there are two struts S6 and S7 which extend from the underside of housing B at approximately 45xc2x0 angles to the side of the shell. These struts are located on opposite sides of strut S5. The struts S5-S7 also are comprised of plates and the three struts are co-planar with each other as installed.
Foundation plates F (only one of which is shown in FIG. 1) are attached to an outer wall W2 of the shell at a location approximately 36 inches (91 cm) below the centerline of shell H. The foundation plates are located opposite of each other. Axial gibs G (again only one of which is shown in FIG. 1) are also installed on the outside of the shell. The gibs, which are also located approximately 36 inches (91 cm) below the centerline of shell H, serve to constrain the structure axially. An access hatch C is located in the top of shell H. An atmospheric relief diaphragm (not shown) is installed in the opening to prevent overpressure conditions from damaging the turbine.
A major problem with this construction is that the support provided by the various struts, plates, and gibs does not consistently provide for reliable startup and operation of the turbine of which the LP section forms a part. That is, the vibration created during turbine start-up will sometimes cause excessive movements in the structure that cause internal damage to the turbine. For example, the support struts have been found not to provide adequate structural integrity. Also, because of the location of the gibs, when a vacuum is drawn within the exhaust casing an undesirable tilting of the LP assembly occurs.
These faults have been demonstrated by extensive field work as well as by finite element analyses (FEA). Besides the various startup problems which occur, there are also losses in performance due to parts rubbing together, steam leakages through the packing rings housed within this section, exhaust pressure loss, and packing rubs.
By redesigning the existing bearing support structure, these problems can be overcome. In addition, exhaust steam pressure recovery in the LP section of the turbine can be improved which results in enhanced performance in this section of the turbine.
Briefly stated, the present invention is directed to a bearing support structure for the LP section of a steam turbine which provides greater stiffness than obtained from current bearing support structures. The bearing support structure significantly differs from previous designs in the number, size, location, and the shape of struts comprising the support structure. By optimizing internal strut shape, quantity, and placement, better and more consistent performance of the turbine, both during startup, and subsequently, is achieved. Further, foundation plates and gibs have been relocated to improve bearing support stiffness and eliminate tilting problems during operation. The resulting structure also minimizes packing ring rubs which otherwise occur because of insufficient support stiffness.
The bearing support structure of the invention has been shown, through the use of finite element analysis, to have substantially improved bearing support stiffness. It has further been shown, through computational fluid dynamics (CFD) analysis, to significantly improve the performance of a steam turbine over turbines having a conventional LP bearing support arrangement. Consequently, steam turbine performance is now improved using the new support design described herein, rather than the support geometry shown in FIG. 1.
Other objects and features will be in part apparent and in part pointed out hereinafter.